At present, substantially two convertiplane configurations are known: “Tilt Rotor”, and “Wing Rotor”.
In the “Tilt Rotor” configuration, the semi-wings remain substantially fixed, and only the motor-rotor assemblies rotate relative to the semi-wings.
Furthermore, the tilt mechanism comprises a pair of actuators for tilting relative rotors and an shaft which connects the actuators.
Examples of “Tilt Rotor” configuration and of relative tilt mechanism are shown in U.S. Pat. No. 6,220,545 or in US-A-2009/0256026.
In the “Tilt Wing” configuration, the rotor attitude is adjusted by rotating the semi-wing and rotors system assembly as a whole. An example of “Tilt Wing” configuration is shown in EP-A-1057724.
Therefore, both in the “Tilt-Rotor” and in the “Tilt-Wing” configuration, the tilting of the rotors is simultaneously.
In both the configuration, each rotor substantially comprises a shaft rotating about its own axis and a plurality of blades which are articulated onto the shaft.
Each blade has a variable angle of attack, i.e. a variable angle between a reference line on such blade and a vector defining the relative motion between the blade and air.
Known convertiplanes comprise a cyclic control for changing the angles of attack of all the blades cyclically during the rotation of the blades about the axis of the shaft. In other words, the angle of attack of the blades changes on the basis of the angular position of the blades relative to the axis of the shaft.
The cyclic control results in the tilting of the rotor disk relative to the fixed axis of the shaft in a particular direction. Accordingly, the tilting of the rotor disk generates a thrust in the particular direction and, therefore, the helicopter advances in the particular direction.
In the known convertiplane, a wide range of maneuvers are carried out through the cyclic control.
For example, in known “Tilt-Rotor” convertiplanes, the yaw is controlled in the helicopter configuration by tilting forward the disk of one rotor and simultaneously tilting backward the disk of the other rotor.
As a result, one rotor generates a thrust in the forward direction and the other rotor generates a thrust in the backward direction.
These two thrusts are parallel and staggered each other, and therefore generates a yawing moment on the convertiplane.
In other words, yaw in hovering is controlled by making use of the cyclic control in the known convertiplanes.
As a result, yaw control results in considerable control moments acting on the shafts of the rotors.
These control moments cause considerable stresses in the shaft, which result in a reduced life-time of the rotors and in a high maintenance cost.
A need is felt within the industry to reduce the stresses generated on the rotors components by the yawing motion, so as to increase the life and reduce the maintenance cost of the rotors.
Furthermore, with reference to known convertiplanes, the maximum yawing moment and, therefore, the promptness of the yawing depends on the maximum variation of the angles of the attack of the rotor blades and on the distance between the axis of the rotors.
Accordingly, the promptness of yawing is penalized by the fact that there is an upper limit to the maximum variation of the angles of attack of the blades.
A need is felt within the industry to increase, as far as possible, the promptness of the yawing in the convertiplane.
Furthermore, also the low speed transition between the helicopter mode and the aeroplane is achieved through the inclination of the rotor disks, i.e. by cyclically varying the angles of attack of the blades.
Finally, known convertiplanes move rearward in a very similar way to traditional helicopters. More precisely, in the helicopter mode, disk rotors are inclined rearward, so as to generate a rearward thrust on the convertiplane.
Also in this case, shafts of the rotors are subjected to considerable control moments, and, therefore, to considerable resulting stresses.
A need is felt within the industry to reduce the stresses generated on the rotors components by the above-identified convertiplane motion, so as to increase the life and reduce the maintenance cost of the rotors.